i'm assuming that it is just one gear that you can not select and that the others are fine, if that is the case it sounds like the "selector fork" for the gear you can not select may be broken. should it be so then the gearbox would have to be removed and dis-assembled, not an easy job to DIY, a replacement gearbox may be a cheaper option as internal gearbox parts can be expensive.
Colin A, February 2005